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The district attorney, Robert Broussard, said one had seemed “arrogant” and “pretty vocal.” In another woman, he said he “detected hostility.”

Mr. Broussard also questioned the “sophistication” of a former Army sergeant, a forklift operator with three years of college, a cafeteria manager, an assembly-line worker and a retired Department of Defense program analyst.

The analyst, he said, “did not appear to be sophisticated to us in her questionnaire, in that she spelled Wal-Mart, as one of her previous employers, as Wal-marts.”

Arguments like these were used for years to keep blacks off juries in the segregationist South, systematically denying justice to black defendants and victims. But today, the practice of excluding blacks and other minorities from Southern juries remains widespread and, according to defense lawyers and a new study by the Equal Justice Initiative, a nonprofit human rights and legal services organization in Montgomery, Ala., largely unchecked.

In the Madison County case, the defendant, Jason M. Sharp, a white man, was sentenced to death after a trial by a jury of 11 whites and one black. The April hearing was the result of a challenge by defense lawyers who argued that jury selection was tainted by racial discrimination — a claim that is difficult to prove because prosecutors can claim any race-neutral reason, no matter how implausible, for dismissing a juror.

While jury makeup varies widely by jurisdiction, the organization, which studied eight Southern states — Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, South Carolina and Tennessee — found areas in all of them where significant problems persist. In Alabama, courts have found racially discriminatory jury selection in 25 death penalty cases since 1987, and there are counties where more than 75 percent of black jury pool members have been struck in death penalty cases.

An analysis of Jefferson Parish, La., by the Louisiana Capital Assistance Center found that from 1999 to 2007, blacks were struck from juries at more than three times the rate of whites.

In North Carolina, at least 26 current death row defendants were sentenced by all-white juries. In South Carolina, a prosecutor said he struck a black potential juror because he “shucked and jived” when he walked.

Studies have shown that racially diverse juries deliberate longer, consider a wider variety of perspectives and make fewer factual errors than all-white juries, and that predominantly black juries are less likely to impose the death penalty.

Excluding jurors based on race has been illegal since 1875, but after Reconstruction, all-white juries remained the norm in the South.

“It really made lynching and the Ku Klux Klan possible,” said Christopher Waldrep, a historian at San Francisco State University and the author of a forthcoming book about a lawyer who was able, in a rare case, to prove jury discrimination in Mississippi in 1906. “If you’d had a lot of black grand jurors investigating crimes, it would have made lynching impossible.”

Back then, judges and prosecutors often argued that blacks lacked the intelligence or education to serve. That such claims persist is evidence, said Bryan A. Stevenson, the executive director of the Equal Justice Initiative, that jury selection remains largely unscrutinized.

“There’s just this tolerance, there’s indifference to excluding people on the basis of race, and prosecutors are doing it with impunity,” Mr. Stevenson said. “Unless you’re in the courtroom, unless you’re a lawyer working on these issues, you’re not going to know whether your local prosecutor consistently bars people of color.”

In jury selection, potential jurors are first dismissed for cause — reasons like scheduling conflicts or opposition to the death penalty. Then, both sides can ask questions and take turns dismissing jurors using what are called peremptory strikes (the number of strikes varies by state, but it is often enough for one side to eliminate all qualified minorities).

In a 1986 case, Batson v. Kentucky, the Supreme Court ruled that if a pattern of discrimination emerged during peremptory strikes, lawyers must provide nonracial reasons for their strikes. The reason does not have to be “persuasive, or even plausible,” the Supreme Court ruled in a later case in which a prosecutor said he dismissed one black juror because he had long hair, and another because he had a goatee, saying, “I don’t like the way they looked.” It is up to the judge to decide if there was deliberate discrimination.

That is a high bar, defense lawyers say — so high that in Tennessee and North Carolina, there has never been a successful reversal based on Batson.

“Anybody with any sense at all can think up any race-neutral reason and get away with it,” said Stephen B. Bright, a capital defense lawyer in Atlanta.